Robustness of circadian clocks to daylight fluctuations: hints from the picoeucaryote Ostreococcus tauri

TL;DR

This study models the circadian clock of Ostreococcus tauri, revealing its robustness to daylight fluctuations and suggesting that light coupling occurs only during specific intervals, minimizing the impact of daylight variability.

Contribution

It presents the first minimal two-gene feedback loop model for Ostreococcus tauri's circadian clock, highlighting its robustness to light fluctuations and the limited coupling to daylight.

Findings

Model accurately reproduces gene expression profiles

Oscillator is not coupled to the diurnal cycle under optimal conditions

Coupling to light occurs only during specific time intervals

Abstract

The development of systemic approaches in biology has put emphasis on identifying genetic modules whose behavior can be modeled accurately so as to gain insight into their structure and function. However most gene circuits in a cell are under control of external signals and thus quantitative agreement between experimental data and a mathematical model is difficult. Circadian biology has been one notable exception: quantitative models of the internal clock that orchestrates biological processes over the 24-hour diurnal cycle have been constructed for a few organisms, from cyanobacteria to plants and mammals. In most cases, a complex architecture with interlocked feedback loops has been evidenced. Here we present first modeling results for the circadian clock of the green unicellular alga Ostreococcus tauri. Two plant-like clock genes have been shown to play a central role in Ostreococcus clock. We find that their expression time profiles can be accurately reproduced by a minimal model of a two-gene transcriptional feedback loop. Remarkably, best adjustment of data recorded under light/dark alternation is obtained when assuming that the oscillator is not coupled to the diurnal cycle. This suggests that coupling to light is confined to specific time intervals and has no dynamical effect when the oscillator is entrained by the diurnal cycle. This intringuing property may reflect a strategy to minimize the impact of fluctuations in daylight intensity on the core circadian oscillator, a type of perturbation that has been rarely considered when assessing the robustness of circadian clocks.